1. Field of the Invention
The present invention relates to a pressure sensor for converting a fluid pressure to an electric signal for measurement.
2. Description of Related Art
Conventionally, there has been used a pressure sensor for measuring pressure of a fluid such as gas and liquid, in which the fluid pressure is converted to an electric signal for measurement. For instance, there is known a pressure sensor disclosed in Reference: Japanese Patent Laid-Open Publication No. 2001-242031. The pressure sensor includes a port attachment portion having a diaphragm and a strain gauge sensor, the pressure sensor measuring pressure of a fluid circulating in a pipe. In the port attachment portion, the diaphragm formed in a bottomed cylindrical shape having a thin-wall portion on a bottom thereof and the pressure introducing joint having a substantially cylindrical shape are integrated. An opening end of the pressure introducing joint is jointed with an end of the pipe in which the fluid circulates, from which the fluid is introduced inside the diaphragm. The strain gauge is provided on an outer surface of the thin-wall portion of the diaphragm with an insulating layer interposed therebetween. When the fluid introduced inside the diaphragm acts on the thin-wall portion, the thin-wall portion is strained due to the pressure of the fluid. The strain gauge detects the strain as an electric signal, and the electric signal is amplified and converted to obtain a pressure value of the fluid.
In such arrangement, in order to detect the pressure value of the fluid with higher accuracy, a higher strain should preferably be generated, and thus the thin-wall portion of the diaphragm should preferable be formed to be thinner. However, since the diaphragm receives a strong pressure from the fluid, high tensile strength is required for a material to be used. With regard to this, a martensitic precipitation hardening stainless steel is used as a material of the diaphragm, and since the material has a tensile strength of 1300 MPa or higher, the pressure value can be obtained with high accuracy and high reliability, which is suitable as a material for the diaphragm of the pressure sensor.
However, although the mechanical strength of the martensitic precipitation hardening stainless steel is high, the corrosion resistance thereof is low, so that the use of the pressure sensor disclosed in the Reference is limited for fluid such as acid gas or chemical solution having high corrosiveness and hydrogen gas causing embrittlement.
In addition, since the martensitic precipitation hardening stainless steel is expensive as well as difficult to machine, size, machining man-hours and scrap amount of members made of such material should preferably be minimized. Thus, if the material is used only for the diaphragm having the thin-wall portion, usage amount of the material can be kept to minimum, and thus the members can be manufactured with relatively low cost. However, the port attachment portion of the pressure sensor disclosed in the Reference has an arrangement in which the diaphragm and the substantially cylindrical pressure introducing joint are integrated. Therefore, in the pressure sensor, since the material is also used for the pressure introducing joint, which is not necessary, an extra cost is required in manufacturing the pressure sensor.